Thermally simulated luminescence (TSL) and electron spin resonance (ESR) studies were carried out on bismuth doped CaS phosphor synthesized by using solid-state reaction technique. The defect centers formed in CaS:Bi were studied using the technique of electron spin resonance. The thermaluminescence glow curve shows two peaks at about 400 and 505 K. Irradiated CaS:Bi exhibits ESR lines due to defect centers. Thermal annealing behaviour of the phosphor indicates that one of the defect centres correlates with the TSL peaks at 400 and 505 K. The center is characterized by an isotropic g-value of 2.0034 and is assigned to a F+ center.